Abstract Liver disease is a cause of substantial morbidity and mortality in the U.S., and cholestasis (impairment of bile flow) is a common and devastating manifestation of liver disease. The focus of the studies proposed in this application is upon increasing our understanding of biological mechanisms that influence the nature and severity of the manifestations of cholestatic liver disease. This work will be accomplished through continued study of a mouse model of FIC1 deficiency. Mutations in the gene encoding FIC1, a protein that plays a role in aminophospholipid transport, underlie some cases of progressive familial intrahepatic cholestasis (PFIC) and benign recurrent intrahepatic cholestasis (BRIC). The severity and manifestations of FIC1 deficiency can vary notably between patients who share the same FIC1 mutation, and we have demonstrated that genetic background influences manifestations of cholestasis in the Fic1-deficient mouse. We have genetically mapped 7 quantitative trait loci (QTL) which influence phenotypes in the Fic1-deficient mouse, and thus, represent modifier genes of the Fic1-deficient phenotype. In Specific Aim 1, we will perform high-resolution genetic mapping studies and comparative haplotype analysis, using existing samples, to refine the localization of these QTL. In Specific Aim 2, we will use a novel, hybrid genetic approach to efficiently and simultaneously further refine the locations of these modifier genes to regions of modest genomic size, thereby decreasing the number of candidate genes that need be considered for each locus;specific genetic variants that are strong candidates for the causative variation underlying some QTLs will also be identified. In Specific Aim 3, gene expression studies and mining of available functional data will contribute to identification of strong QTL candidate genes. Studies proposed in Specific Aim 4 will yield increased understanding of specific phenotypes detected in the Fic1-deficient mouse, and related QTLs. Identification of the genetic variants underlying such QTL will a) shed light on mechanisms of disease in FIC1 deficiency;b) identify candidate modifier genes for FIC1 deficiency, and possibly other liver disorders, in people;and c) provide information of use to the broad community of researchers studying mouse models of liver disease, regarding susceptibility of the commonly used mouse strains to liver disease-related phenotypes.